In the last 15 years or so, the literature has been replete with descriptions of various microcrystalline or non-vitreous fibers and other shaped objects prepared from refractory metal oxides which objects are made by various non-melt processes. One process that has received a fair amount of attention in recent years is the production of fibers by extruding, drawing or spinning viscous fluids prepared from metal or metalloid alkoxides, which production processes are often referred to as sol gel processes. In 1982, it was described how sols and high-purity, dense microparticles of a variety of ceramic oxides could be prepared from alkoxides, K. S. Mazdiyasni, Ceramics International, 8, 2, 42-55 (1982). Fibers or other extruded objects prepared from such sols are generally dried at intermediate temperatures to remove solvents and other volatile organic substances and then fired at high temperatures to calcine the remaining organic materials to produce the desired refractory articles. With respect to making crystalline fibers from organic precursors, U.S. Pat. Nos. 4,732,878 and 4,924,623, and the art cited in those patents, are pertinent, disclosing various processes for making crystalline fibers of this general type from sols.
Most recently, there has been a search for such high-temperature-resistant fibers that can be used to endow ceramic composite materials and the like with good creep resistance at such high temperatures. Although there are a variety of fibrous ceramic materials available from a number of different commercial manufacturers that are generally useful for ceramic matrix reinforcement, none of these fibers has all of the properties desired, and thus the search continues for improved fibers that exhibit high strength, high temperature resistance and particularly good creep resistance. Although oxide-based ceramics have continued to be shown much interest because of their inherent oxidation-resistance at high temperatures, other non-oxide ceramic fibers have thus far shown superior creep resistance and thus have been the focus of continuing development because of this favorable property,
A number of oxide fibers have been developed and are now commercially available, including the FP family of fibers marketed by DuPont, the Nextel family of fibers marketed by Minnesota Mining and Manufacturing, Saffil fibers available from ICI, and Saphikon fibers from Saphikon and Sumika fibers from Sumitomo. However, until fairly recently, such available oxide fibers have tended to chemically bond tightly to the ceramic matrices, and they have been inappropriate for this reason because composites would behave more like monolithic ceramics and would fail under stress in a brittle-like manner. Because coatings for such fibers have progressed, new interest has been shown in oxide fibers for these purposes, and presently both Nextel 312 fibers and fiber FP are available with silicon dioxide coatings. However, even though such improved coatings alleviate the problem with regard to brittleness, creep resistance continues to remain a significant problem for these fibers.
One process often used for preparing high purity mullite powder (3Al.sub. O.sub.3.2SiO.sub.2) is that of hydrolytically decomposing a mixture of aluminum and silicon alkoxides, wherein the hydrolytic condensation reactions are carried out in the presence of ammonium hydroxide or in the presence of dilute mineral acid with the formation of a hydroxy-aluminosilicate. Such material can be caused to precipitate in the form of white power which can then be dried and fired to produce polycrystalline mullite.
The '878 patent teaches making mullite fibers from an alkoxide mixture of this general type by creating a sol that, after appropriate adjustment of viscosity, is extruded or spun to create fibers; these green fibers are then dried and calcined or fired to achieve the ceramic crystalline form. An article by S. Sakka et al., J. Non-Crystalline Solids, 48, 31-46 (1982) discussed catalysts for alkoxide condensation reactions to produce a viscous liquid sol useful to spin silica and silica-based fibers.
In addition to mullite-based and other silica-based fibers, the search continued to high-strength oxide fibers to reinforce ceramic and metal matrix composites, and in 1987, there was a publication entitled High-Strength Zirconia Fibers, D. D. Marshall, et al., J. Am. Ceram, Soc., 70, 8, C-187-C-188 (1987). This article reported the fabrication of high-strength Y.sub.2 O.sub.3 -doped ZrO.sub.2 fibers from a metastable, acetate-based precursor. The 1990 publication Fiber Reinforced Ceramic Composites, edited by K. S. Mazdiyasni, Noyes Publications, includes Chapter 5, entitled "Oxide Fibers from Chemical Ceramic Processes", wherein many such processing techniques are surveyed.
Despite these early successes, the search has still continued for improved ceramic oxide fibers and particularly for fibers that would exhibit good creep resistance at high temperatures so as to facilitate their use as reinforcing fibers in ceramic composite materials. It was well known that yttrium alumina garnet (YAG), Y.sub.3 Al.sub.5 O.sub.12, was a material that exhibits long-term thermochemical stability and reasonable mechanical properties at high temperatures. In a 1984 article, G. de With, Mat, Res. Bull., 19, 1669-1674, reported that spray-dried particles of YAG could be sintered to dense, translucent shapes using dopants such as SiO.sub.2, MgO or possibly ZrO.sub.2. However, the production of YAG fibers having high temperature stability and good creep resistance has not previously been achieved.
In view of the foregoing, efforts have continued which are directed to the production of microcrystalline YAG material having improved creep resistance while retaining other desired high temperature properties, with particular emphasis being focused on the production of oxide fibers which not only will exhibit good heat resistance, but which can be used as reinforcing fibers to impart high creep resistance to ceramic composite materials.